Epidemiological studies show significant associations between exposure to particulate matter with particles of aerodynamic diameter of <_2.5 [unreadable]m (PM2.5) and cardiovascular-related morbidity including ventricular arrhythmias and sudden cardiac death. While there appears to be little doubt that PM2.5 exposure poses a significant cardiovascular health risk, the underlying causes are poorly understood. The decreased heart rate variability (HRV) associated with PM2.5 exposure is particularly important since decreased HRV has been shown to be an index of cardiac vagal regulation and is associated with increased susceptibility to ventricular arrhythmias and risk for cardiovascular-related sudden death. Even less understood is the mechanism(s) mediating the reduced HRV and hence the cardiovascular-related morbidity. Using the mouse we propose to use state-of-the-art inhalation facilities to deliver environmentally relevant particulate pollutants (iron/soot) from a true combustion source that captures the carbon-based particles and a transition metal ubiquitous in the environment to test the Hypothesis that short-term (3-day) exposure to PM2.5 results in a reduced HRV due to decreases in the intrinsic membrane properties and/or synaptic excitability of anatomically- and functionally-identified CNS cardiac vagal neurons in the nucleus ambiguous (NA) that regulate HRV. We will test the hypothesis in mice exposed to two concentrations of iron/soot particles and filtered air (FA) as a control by the following Specific Aims. 1. To determine whether short-term (3-day) exposure to PM2.5 in the form of iron/soot particles produces the phenotype of a reduced cardiac vagal regulation of heart rate, by quantifying overall 24-h HRV, diurnal changes in HRV, and heart rate recovery following an acute stressor (exercise). 2. To determine whether the PM2.5 exposure-induced decrease in HRV is mediated by decreased intrinsic excitability of the NA cardiac vagal neurons by measuring resting membrane potential, membrane conductance and spiking responses to depolarizing current injections. 3. To determine whether the PM2.5 exposure-induced decreased intrinsic excitability of NA cardiac vagal neurons is mediated by increased potassium currents, left shift in activation kinetics, and/or right shift in inactivation kinetics of three major potassium channels present in NA neurons. 4. To determine whether the PM2.5 exposure-induced decrease in HRV is mediated by decreased synaptic excitability by enhanced inhibitory ;?-aminobutyric acid (GABA) mechanisms at the NA cardiac vagal neurons, by measuring the frequency and amplitude of tonic GABA-mediated inhibitory postsynaptic currents (GABA IPSCs). 5. To determine whether the PM2.5 exposure-induced decrease in HRV is mediated by decreased synaptic excitability by depressed glutamatergic (GLU) excitatory mechanisms at the NA cardiac vagal neurons, by measuring the frequency and amplitude of tonic excitatory postsynaptic currents (GLU EPSCs) and the amplitude of evoked GLU EPSCs. [unreadable] [unreadable]